Those skilled in the art are aware of vehicle wheel bearings having sealing devices with integrally mounted magnetic encoders to detect the rotation speed of the wheels. FIG. 8 is a cross-section view of a prior art vehicle wheel bearing. An inner member 50, an outer member 60, and double row rolling elements 70 and 70 are arranged between the inner and outer members 50 and 60. The inner member 50 has a wheel hub 51 and an inner ring 52 fitted on the wheel hub 51. The wheel hub 51 is formed integrally with a wheel mounting flange 53 to mount a wheel (not shown). Hub bolts 54, which secure the wheel, are equidistantly arranged along the periphery of the flange 53. The inner ring 52 is press-fitted on a stepped portion 55, of small diameter, formed on the wheel hub 51. The end of the stepped portion 55 is plastically deformed radially outward to form a caulked portion 56 to prevent the inner ring 52 from being axially moved from the wheel hub 51.
The outer member 60 is formed integrally with a body mounting flange 61 to be mounted on a knuckle “N” at its periphery. The outer member 60 also has double row outer raceway surfaces 60a and 60a at an inner circumferential surface opposite to the inner raceway surfaces. In the inner member 50, double row inner raceway surfaces 51a and 52a are formed on the outer circumferential surfaces of the wheel hub 51 and the inner ring 52, respectively, opposite of the double row outer raceway surfaces 60a and 60a of the outer member 60. Double row rolling elements (balls) 70 and 70 are arranged between the outer and inner raceway surfaces 60a and 51a; 60a and 52a. The rolling elements are held to be freely rolled by cages 71 and 71. Sealing devices 62 and 63 are mounted on the ends of the outer member 60 to prevent leakage of lubricating grease contained within the bearing. The sealing devices 62 and 63 also prevent penetration of rain water or dusts into the bearing from ambient. Bearings having such a structure are called “third generation”.
In such a vehicle wheel bearing, the sealing device 63, as clearly shown in an enlarged view of FIG. 9, has first and second annular sealing plates 64 and 65 mounted on the inner ring 52 and the outer member 60, respectively. Each of these sealing plates 64 and 65 has a substantially “L”-shaped cross-section formed respectively by a cylindrical portion 64a and 65a and a radially extending portion 64b and 65b. The sealing plates 64 and 65 are arranged opposite to each other. The radially extending portion 64b of the first sealing plate 64 has an encoder 66 to detect the wheel rotation speed. The encoder is bonded, via vulcanization, at inboard side of the bearing. The encoder 66 is made of a rubber magnet where magnetic substance powder is mingled in the material and N and S poles are alternately magnetized along the circumferential direction of the encoder.
The second sealing plate 65 has a sealing member 67 bonded to it via vulcanization. An integrally formed side-lip 67a slide-contacts with the radially extending portion 64b. The other integrally formed radial-lips 67b and 67c slide-contact with the cylindrical portion 64a. A tip of the radially extending portion 64b of the first sealing plate 64 is opposed to the cylindrical portion 65a of the second sealing plate 65 with a slight radial gap formed therebetween to provide a labyrinth seal 68.
The first sealing plate 64 forms a slinger and is press-fitted on the outer circumferential surface of the inner ring 52, however, traces of water may penetrate into the inside of bearing through the fitted portion. Penetration of water not only degrades the lubricating grease, but reduces the life of the bearing through the generation of rust in the first sealing plate 64 and thus wear of the sealing lips.
Applicant of the present invention has proposed in Japanese Laid-open Patent Publication No. 215132/2001, a sealing structure shown in FIG. 10, to overcome this problem. According to this sealing structure, a projected piece 66a, extending from the encoder 66, elastically contacts the caulked portion 56 to seal an exposed surface 69 (see FIG. 9) of the inner ring 52. The piece also prevents water from penetrating into the bearing through the abutted surface between the end face of the inner ring 52 and the caulked portion 56.
Since the rubber magnet member forming the encoder 66 is mingled with a great deal of magnetic powder, it becomes not only expensive but lacks appropriate elasticity. In addition, hoop stress is sometimes caused between the fitted surfaces of the inner ring 52 and the sealing plate 64 by the radial plastic deformation of the stepped portion 55 due to caulking working. If corrosion occurs in this fitted portion under such circumstances, the diffusible hydrogen existing in nature will penetrate into the metallographic structure of the inner ring 52 and destroy the metallic grain bond and cause undesirable so-called “delayed fracture”.
In addition, since the surface of the encoder 66 is exposed to the circumstance and arranged opposite to a speed sensor (not shown), via a predetermined gap, dusts or sand, penetrates into the gap and wears their surface. Increase of the air gap between the encoder 66 and the speed sensor, due to wear, causes reduction of the accuracy of detection.